1. Field of the Invention
The present invention generally relates to power converters; more specifically, the present invention relates to quasi-resonant power converters.
2. Description of the Related Art
To improve efficiency and reduce power loss at high switching frequency, a quasi-resonant power converter is a solution. The technical details of quasi-resonant power converter can be found in the prior art “Switching Control Circuit Having a Valley Voltage Detector to Achieve Soft Switching for a Resonant Power converter”, U.S. Pat. No. 7,426,120. However, the drawback of the quasi-resonant power converter is lower efficiency under light-load conditions. A switching frequency of the quasi-resonant power converter is varied in response to an input voltage and an output load thereof. Once the input voltage increases or the output load decreases, the switching frequency of the quasi-resonant power converter will inherently increase. The increased switching frequency under light-load conditions will increase the switching loss of the quasi-resonant power converter and therefore reduces the efficiency.